1. Field of the Invention
The invention relates to a method of measuring the dimensions of the optical focus of an X-ray tube from a radiogram obtained by means of a pinhole camera, and is applicable especially to the radiographic inspection of components.
It is common practice to use radiographic inspection to detect internal defects in a component. These defects may arise from a lack of material or the presence of materials of different densities. In the case of an X-ray photograph or radiogram, the regions lacking in material form darker areas in the radiogram, due to lower absorption of the radiation, and the presence of denser or less dense materials gives, respectively, lighter or darker areas in the radiogram, due to different absorption of the radiation. However, this method does not enable defects below a certain minimum size to be detected, this minimum size depending on the sensitivity of the detector, on the power of the X-ray tube, and on the dimensions of the optical focus of the X-ray tube. In order to detect defects in a component and to measure their dimensions, it is therefore necessary to know the characteristics of the X-ray tube, and in particular to know the dimensions of the optical focus of this tube.
Moreover, since the size of the optical focus and the homogeneity of the emitted beam may vary over time, it is necessary to be able to monitor the dimensions of this focus and to monitor the homogeneity of the emitted beam in the case of doubt if the quality of the images obtained has become degraded, and even to carry out these monitoring operations periodically in order to ensure that there is no drift.
2. Summary of the Prior Art
In order to measure the dimensions of the-optical focus of an X-ray tube, it is known to use a pinhole camera placed in the path of the rays coming from the anode of an X-ray tube to produce an X-ray image of the optical focus on a fine-grained X-ray-sensitive film. By suitably choosing the distances between the pinhole and the plane of the film and between the focus of the tube and the pinhole, an enlarged image of the focus is obtained on the film. The image thus obtained may be examined, in a known manner, by eye through a graduated magnifying glass or by using a microdensitometer.
When examining the image by eye, the dimensions of the focus are measured from the perceptible edges of the image and by taking into account the magnification factor. However, since this method is subjective, it is not reliable and leads to results differing from one operator to another.
When examining the image by means of a microdensitometer, the measurement is carried out automatically. The microdensitometer calibrates, makes a section along a single line of the image and measures the length of the optical focus on the basis of this section. This method is very accurate as long as the microdensitometer is positioned correctly so that the section is made at the place where the spot physically representing the optical focus is widest. However, since the section thus made enables only a single image line to be seen at a time, this method does not make it possible to know the energy distribution nor to check the homogeneity of the emitted beam. The energy distribution can be obtained only by carrying out a complete scan of the radiogram, which takes up a lot of time and is very expensive.